fix io_uring max submissions being capped to 0

[catfinated]

This addresses #1949.

With this change, the io_uring context submission queue max submissions no longer gets capped to 0 which allows work to actually be submitted to the kernel. In particular, the bug this is addressing was causing the run_until_stopped() method to break out of the main loop immediately causing both the example code and unit test to hang indefinitely at blocking sync_wait calls.

This also resolves an issue in the io_uring related unit tests where calls to start_detached were ambiguous. This was necessary to compile with io_uring support enabled as well as to validate the fix since the unit test also encountered the hanging issue.

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[ericniebler]

how funny. i spotted that __umin bug this morning and just fixed it. are the other changes still necessary? CI is green.

[catfinated]

That is funny. I did post an issue report a few days ago that included a fix. I am particularly interested in the io_uring support so it was the first thing I tried to run and ran into that bug.

As far as the other changes I believe they are needed but its possibly compiler dependent (or I am possibly missing some detail as I just started looking at the codebase). I am on gcc 15.2.1 and clang 22.1.1 and without those changes to the unit test I get an ambiguous function call error:

[78/141] Building CXX object test/exec/CMakeFiles/test.exec.dir/test_io_uring_context.cpp.o
FAILED: [code=1] test/exec/CMakeFiles/test.exec.dir/test_io_uring_context.cpp.o 
/usr/bin/c++ -DSTDEXEC_NAMESPACE=std::execution -I/ssd1/projects/oss/stdexec/include -I/ssd1/projects/oss/stdexec/build/include -I/ssd1/projects/oss/stdexec/build/_deps/catch2-src/single_include -I/ssd1/projects/oss/stdexec/test -O3 -DNDEBUG -std=gnu++20 -fcoroutines -fconcepts-diagnostics-depth=10 -Wall -Werror=unused-parameter -Wno-non-template-friend -MD -MT test/exec/CMakeFiles/test.exec.dir/test_io_uring_context.cpp.o -MF test/exec/CMakeFiles/test.exec.dir/test_io_uring_context.cpp.o.d -o test/exec/CMakeFiles/test.exec.dir/test_io_uring_context.cpp.o -c /ssd1/projects/oss/stdexec/test/exec/test_io_uring_context.cpp
/ssd1/projects/oss/stdexec/test/exec/test_io_uring_context.cpp: In function ‘void {anonymous}::____C_A_T_C_H____T_E_S_T____6()’:
/ssd1/projects/oss/stdexec/test/exec/test_io_uring_context.cpp:122:5: error: reference to ‘start_detached’ is ambiguous
  122 |     start_detached(schedule(scheduler)
      |     ^~~~~~~~~~~~~~
In file included from /ssd1/projects/oss/stdexec/include/exec/finally.hpp:18,
                 from /ssd1/projects/oss/stdexec/test/exec/test_io_uring_context.cpp:24:
/ssd1/projects/oss/stdexec/include/exec/../stdexec/__detail/__execution_fwd.hpp:323:33: note: candidates are: ‘const experimental::execution::start_detached_t experimental::execution::start_detached’
  323 |   extern start_detached_t const start_detached;
      |                                 ^~~~~~~~~~~~~~
In file included from /ssd1/projects/oss/stdexec/include/exec/linux/../../stdexec/execution.hpp:82,
                 from /ssd1/projects/oss/stdexec/include/exec/linux/io_uring_context.hpp:26,
                 from /ssd1/projects/oss/stdexec/test/exec/test_io_uring_context.cpp:25:
/ssd1/projects/oss/stdexec/include/exec/linux/../../stdexec/__detail/__deprecations.hpp:78:51: note:                 ‘constexpr const experimental::execution::start_detached_t& std::execution::start_detached’
   78 |   inline constexpr exec::start_detached_t const & start_detached = exec::start_detached;
      |                                                   ^~~~~~~~~~~~~~
/ssd1/projects/oss/stdexec/test/exec/test_io_uring_context.cpp: In function ‘void {anonymous}::____C_A_T_C_H____T_E_S_T____16()’:
/ssd1/projects/oss/stdexec/test/exec/test_io_uring_context.cpp:209:7: error: reference to ‘start_detached’ is ambiguous
  209 |       start_detached(schedule(scheduler)
      |       ^~~~~~~~~~~~~~
/ssd1/projects/oss/stdexec/include/exec/../stdexec/__detail/__execution_fwd.hpp:323:33: note: candidates are: ‘const experimental::execution::start_detached_t experimental::execution::start_detached’
  323 |   extern start_detached_t const start_detached;
      |                                 ^~~~~~~~~~~~~~
/ssd1/projects/oss/stdexec/include/exec/linux/../../stdexec/__detail/__deprecations.hpp:78:51: note:                 ‘constexpr const experimental::execution::start_detached_t& std::execution::start_detached’
   78 |   inline constexpr exec::start_detached_t const & start_detached = exec::start_detached;
      |                                                   ^~~~~~~~~~~~~~
[95/141] Building CXX object test/exec/CMakeFiles/test.exec.dir/test_split.cpp.o
ninja: build stopped: subcommand failed.

[catfinated]

[nit] #include <limits> is no longer need with this initializer

[ericniebler]

could you pls run clang-format-21 over test_io_uring_context.cpp?

[catfinated]

@ericniebler Thanks, not sure why my clang-format did the wrong thing. Should I change the PR title to align with the fact that this now just addresses the unit test function call ambiguity issue instead of the __umin bug?